This invention concerns a device for testing the shear strength of a bond in a semiconductor device, and more particularly the strength of a bond between a substrate and a means of electrical connection thereto, typically a part-spherical deposit of solder or gold.
Semiconductor devices are very small, typically from 0.2 mm square to 25 mm square. These devices have sites for the bonding of electrical conductors thereto. Sites typically comprise part spherical deposits of gold or solder, collectively known as balls, which in use have the appearance of a squashed sphere or low circular dome, and a diameter in the range 50-1000 μm. These deposits form part of the electrical path between, for example, a printed circuit board and a chip, and may directly connect components, or may be joined to a conductor which is itself connected to another component. Many such balls may be provided as a regular grid-like array on a substrate.
Discrete balls are typically applied to a substrate and reflowed during subsequent connection to another component.
It is necessary to test the mechanical strength of the bond between the gold or solder deposit and the substrate in order to give confidence that the bonding method is adequate, and that the bond strength is sufficient. Difficulties arise because of the very small dimensions of the components, the precision with which the testing device must be positioned, and the very small forces and deflections which are to be measured.
It has been proposed to test the shear strength of such deposits by applying a tool to one side thereof. In order to avoid friction caused by the tool rubbing on the surface of the substrate, it is necessary for the tool to be just above the substrate surface. The height of the tool above the substrate must be closely controlled, typically within ±0.001 mm, to give accurate force measurement.
A known shear test apparatus comprises a machine having a support surface and a test head movable in a controlled manner relative to the support surface. The test head carries a cartridge specific to the test to be performed and having one of several interchangeable tools thereon. Typically the tool will be sized and/or shaped to suit the ball deposit to be tested. In use the substrate to be tested is attached to the support surface, and the tool is driven relatively against the ball deposit to perform the required test, which may be for example a shear test or a reciprocating fatigue test.
It will be understood that a typical tool is very small, and accordingly the cartridge has a flexible element on which is mounted one or more force gauges (such as strain gauges). Thus shear force between the tool and ball deposit is measured at a distance by deflection in the cartridge.
In the case of impact testing, where the tool is moving at high velocity before contact with the ball deposit, shear forces are not easy to detect. This is because the strain gauged element is somewhat remote from the tool, and the inertia of the cartridge masks the forces being measured. Typically the speed of the test is sufficiently high that the test is over before the strain gauge has time to respond to the forces at the tool.